package main

import (
	"archive/tar"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
)

// Archive is a snapshot read back: its manifest, and each file beside it unpacked into a scratch
// directory of its own, removed by Close.
type Archive struct {
	Manifest Manifest
	dir      string
	files    map[string]string
}

// ReadArchive unpacks a snapshot. Only the files the manifest names are kept, and only under names
// that are one stream's directory: an archive is data, and a path in it is never trusted to say where
// on the machine a file goes.
func ReadArchive(r io.Reader, scratch string) (*Archive, error) {
	dir, err := os.MkdirTemp(scratch, "mesh-nats-restore-*")
	if err != nil {
		return nil, err
	}
	a := &Archive{dir: dir, files: map[string]string{}}
	tr := tar.NewReader(r)
	first := true
	for {
		hdr, err := tr.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			a.Close()
			return nil, fmt.Errorf("the archive does not read: %w", err)
		}
		if first {
			if hdr.Name != ManifestName {
				a.Close()
				return nil, fmt.Errorf("the archive starts with %q, not its manifest: it is not a snapshot of the bus", hdr.Name)
			}
			raw, err := io.ReadAll(io.LimitReader(tr, 64<<20))
			if err != nil {
				a.Close()
				return nil, err
			}
			if err := json.Unmarshal(raw, &a.Manifest); err != nil {
				a.Close()
				return nil, fmt.Errorf("its manifest: %w", err)
			}
			if a.Manifest.Format != 1 {
				a.Close()
				return nil, fmt.Errorf("its manifest is format %d; this program reads format 1", a.Manifest.Format)
			}
			first = false
			continue
		}
		if !a.named(hdr.Name) {
			a.Close()
			return nil, fmt.Errorf("the archive holds %q, which its manifest does not name", hdr.Name)
		}
		local := filepath.Join(dir, fmt.Sprintf("%04d", len(a.files)))
		f, err := os.OpenFile(local, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0o600)
		if err != nil {
			a.Close()
			return nil, err
		}
		_, err = io.Copy(f, tr)
		f.Close()
		if err != nil {
			a.Close()
			return nil, err
		}
		a.files[hdr.Name] = local
	}
	if first {
		a.Close()
		return nil, errors.New("the archive is empty")
	}
	for _, s := range a.Manifest.Streams {
		if !validName(s.Name) || s.Meta != "streams/"+s.Name+"/backup.json" || s.Archive != "streams/"+s.Name+"/stream.tar.s2" {
			a.Close()
			return nil, fmt.Errorf("the manifest names a stream %q at paths that are not its own", s.Name)
		}
	}
	return a, nil
}

func (a *Archive) named(name string) bool {
	for _, s := range a.Manifest.Streams {
		if name == s.Meta || name == s.Archive {
			return true
		}
	}
	return false
}

// Close removes the unpacked files.
func (a *Archive) Close() { os.RemoveAll(a.dir) }

// Verify holds every file to the manifest's size and checksum, and reads every stream's archive to
// its end.
func (a *Archive) Verify() error {
	for _, s := range a.Manifest.Streams {
		for _, f := range []struct{ name, sum string }{{s.Meta, s.MetaSHA256}, {s.Archive, s.ArchiveSHA256}} {
			local, ok := a.files[f.name]
			if !ok {
				return fmt.Errorf("%s is named in the manifest and missing from the archive", f.name)
			}
			got, size, err := sha256Of(local)
			if err != nil {
				return err
			}
			if got != f.sum {
				return fmt.Errorf("%s does not match the checksum its manifest gives", f.name)
			}
			if f.name == s.Archive && size != s.ArchiveBytes {
				return fmt.Errorf("%s is %d bytes; its manifest says %d", f.name, size, s.ArchiveBytes)
			}
		}
		af, err := os.Open(a.files[s.Archive])
		if err != nil {
			return err
		}
		err = readsThrough(af)
		af.Close()
		if err != nil {
			return fmt.Errorf("%s does not read: %w", s.Archive, err)
		}
	}
	return nil
}

func sha256Of(path string) (string, int64, error) {
	f, err := os.Open(path)
	if err != nil {
		return "", 0, err
	}
	defer f.Close()
	h := sha256.New()
	n, err := io.Copy(h, f)
	if err != nil {
		return "", 0, err
	}
	return hex.EncodeToString(h.Sum(nil)), n, nil
}

// open is one stream's meta and archive.
func (a *Archive) open(s StreamEntry) (meta []byte, archive *os.File, err error) {
	meta, err = os.ReadFile(a.files[s.Meta])
	if err != nil {
		return nil, nil, err
	}
	archive, err = os.Open(a.files[s.Archive])
	return meta, archive, err
}